CANADIAN ARCTIC 



645 



Nature of Crust under Deep Basin 



A seismic surface wave of unusually large amplitude (Lg wave) was 

 recognized by Press and Ewing in 1952, and it was noted to have the 

 characteristics of traveling only over paths of continental structure. It 

 does not propagate across oceanic crust. Oliver et al. ( 1955 ) subsequently 

 studied the Lg wave paths across the Arctic region and concluded that 

 the Arctic basin was floored by ocean crust, and that it could not be 

 sunken continental crust as had been postulated by Soviet geologists 

 and Eardley. The subsidence theory will be considered later. 



Figure 40.8 is an interpretation of the crustal constitution across the 

 Arctic basin from Franz Josef Land to Alaska by Demenitsckya (1958). 

 A thickening of the basaltic layer under both the Lomonosov and Alpha 

 ranges is conspicuous, as well as the existence of a 5-kilometer thick 

 "granitic" lens under each. The crust under the basins is typically oceanic. 



Theories of the Origin of the Arctic Basin 



Permanency of the Basin. In about 1860 James D. Dana began to 

 teach that the continents and ocean basins are permanent features of the 

 earth's crust. He contended that in the main the ocean basins have been 

 sinking and the continents rising, but several continental fragments have 

 subsided. Fifty years later Charles Schuchert (1916) in his studies of 

 paleogeography was foremost in supporting Dana. He said: 



Now, however, geologists are holding more and more to the hypothesis that 

 the earth periodically shrinks, and each time it does so some parts or all of the 



0- 



FRANZ JOSEF LOMONOSOV MENDELEEV 

 LAND RANGE RANGE ALASKA 



"~~'Jt^ NANSEN BASIN /s^MAKAROV B^-r- rr-. rr— -^. BEAUFORT BASIN ^-rr..-- ■■^i.-'Si- 



10- 



GRANITE '"--^•--------■-^ *""~~11d_ GRANITE ^Z-- ===£ — " VLv_V__"" GRANITE 





yS >V ^— ^BASALT ^S ^^^ 



20- 



/ MANTLE \^S > ' MANTLE ^^>v 



30- 



^ ^V 



40- 



1000 KM 





Fig. 40.8. Crustal structure of the Arctic basin, after Demenitsckya, 1958. Basin names 

 and Mendeleev Range name, have been added according to information from V. N. Sachs 

 personal communication. Mendeleev Range has been called the Alpha Range by Americans. 



continents rise more or less; but that in the main there is subsidence of tin- ocean 

 bottoms equal in amount to the rising land-masses, that the water of the hydro- 

 sphere is constandy increasing in amount, and that even though the continent! 

 are in the main permanent, yet they are partially breaking down into the oceanic 

 basins. 



Reference is made by Schuchert to the permanency of the North 

 Atlantic Ocean basin, but we can only presume that he considered the 

 Arctic Ocean basin a permanent feature; his maps are not definitive 

 about the Arctic. 



Subsidence Theory. In the period of 1930 to 1950 and beyond, the 

 Russians, beginning with Shatski (Hope, 1959a) considered the Arctic 

 Ocean basin as a sunken region, once emergent. The sunken crust was 

 called the Hyperborean shield, and the depression as the Hyperborean 

 basin. Later, associates of Shatski referred to the shield as a massif or 

 platform. The sunken platform was postulated as a result of an en- 

 visioned belt of Mesozoic folding encircling the basin, as a once resistant 

 shield. 



A development of the thesis that the fold belts which extend to the 

 Arctic Ocean cross the shelves and deep basin is shown in Fig. 40.9. 

 Here, Sachs et al. (Hope, 1959a) show Caledonian, Hercynian, and 

 Alpine fold belts extending across the deep basin, particularlv where the 

 Lomonosov Range and Nansen Basin (C) now exist. They postulate, of 

 necessity, that the fold belts have sunken to form the deep basins. 



In 1948 Eardley reviewed the geology of the lands around the Arctic 

 Ocean basin and concurred with the Russians that the basin was a sunken 

 region which in Precambrian and perhaps early Paleozoic times had been 

 land. The broad shelves and relatively small size of the basin, the facing 

 Precambrian shields (Canadian, Greenland, Russian-Baltic, and Angara), 

 the Paleozoic orogenic belts that project to and under (?) the Arctic 

 Ocean (Ural-Nova Semlya, Norway and Spitzbergen, East Greenland, 

 Canadian Arctic Archipelago, Northland, and New Siberian) suggested 

 to him that the region was once land and beginning in Paleozoic time 

 has foundered. Paleozoic geosynclinal conditions in Alaska seemed to 

 require a source for some of the sediments north of land today, where 

 water is fairly deep. Paleozoic fossil faunas common to North America 



